In taking a drug, oral ingestion is the simplest form of administration. However, this route of intake is applicable mostly to low molecular weight compounds which are relatively stable at the pH of the stomach and gastrointestinal tract. Life-saving drugs such as insulin for the treatment of diabetes mellitus, and heparin for anticoagulation in cardiovascular diseases, are macromolecules which are susceptible to proteolytic enzymes or cannot be absorbed through the intestinal wall, if taken orally. Therefore, these drugs can only be administered by injection which imparts too high an initial concentration that may decrease rapidly with reduced therapeutic efficiency. The alternative is to infuse a dilute solution of the drug over the period of time during which therapy is desired.